PYCBC-146: Don't crash on OBSERVE with replicas

Change-Id: Ibcc2b032f321f11497cad8eb7d4cb8c005dde5f4
Reviewed-on: http://review.couchbase.org/27177
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Mordechai Nunberg <mnunberg@haskalah.org>